The present invention relates to a low-density polyethylene resin for laminate, a composition thereof, a laminate using the same and a production method therefor. The laminate is useful for applications to, for example, food wrapping materials, containers, etc.
The present application is based on Japanese Patent Application No. Hei 9-267463, the contents of which are incorporated herein by reference.
As a wrapping material, in particular a food wrapping material, laminates have been used, including substrates made of polymers such as polypropylene, polyamide, polyester, saponification products of ethylene-vinyl acetate copolymer or the like, aluminum foil, cellophane, papers or the like and ethylene-based polymers such as polyethylene, ethylene-vinyl acetate copolymer, for imparting a heat sealing property or water vapor barrier property, the polymer being extrusion laminated on the substrate.
However, polyethylene is a non-polar resin and its adhesion to the substrate is essentially low.
Hence, to increase the adhesion to the substrate, generally, upon extrusion in extrusion laminate molding, the temperature of the resin is set to a high temperature of 310xc2x0 C. or higher to oxidize the surface of the molten thin film extruded from an extruder.
However, when the temperature of resin is set to a high temperature of 310xc2x0 C. or higher, thermal decomposition or oxidative deterioration occurs in polyethylene. As a result, sometimes, a large amount of smoke is produced so that the working environment is adversely affected, or the heat sealing performance of laminate is decreased an odor remains in the laminate, which decreases the product quality.
Further, recently, particularly for increasing productivity, it has been desired to perform molding at higher speed. However, increasing the molding speed further decreases the adhesion strength.
In the case of ethylene-vinyl acetate copolymer, if the extrusion molding temperature is set to 280xc2x0 C. or higher, the ethylene-vinyl acetate copolymer in an extruder or die tends to decompose, thus making the acetic acid odor stronger, or forming bubbles, so that the temperature of resin upon extrusion laminate molding must be 260xc2x0 C. or lower.
However, at such a low temperature, the adhesion strength to the substrate is too low to be practically acceptable.
Accordingly, the measure is taken that polyethylene is extrusion laminate molded on a substrate at a high temperature of 310xc2x0 C. or higher in advance and an ethylene-vinyl acetate copolymer is extrusion laminate molded on the polyethylene surface at a temperature of 260xc2x0 C. or lower. Therefore, the process is complicated and economically disadvantageous.
Japanese Patent Application, First Publication No. Sho 57-157724 discloses a method in which an ethylene-based resin is extruded at a low temperature of 150 to 290xc2x0 C., treated with ozone, the treated surface thereof is contact laminated on an anchor-coat treated substrate.
This method alleviates the smoking or odor because of a low temperature molding. However, a decrease in the molding temperature results in a decrease in the bond strength. Therefore, to ensure a practically acceptable bond strength, the molding speed must be lowered so that there remain great problems in productivity and economics, for example, such that film thickness cannot be made thinner.
Japanese Patent Application, First Publication No. Hei 4-306209 discloses, as a material having an increased adhesion strength, an ethylene-propylene copolymer for extrusion laminates which has a methyl branch number of 3.5 or more per 1,000 carbon atoms in the polymer chain, a melt flow rate in the range of 1 to 20 g/10 min., and a long chain branching index in the range of 0.3 to 0.9. However, a further improvement in adhesion strength by low-temperature, high-speed molding has been desired.
As stated above, for laminates obtained by extrusion lamination molding polyethylene, elevation of the resin temperature in extrusion lamination molding causes the problems of the occurrence of smoking or odor and hence elevation of the temperature cannot be adopted and hence it is difficult to increase the adhesion strength. In particular, it is very difficult to increase the adhesion strength in high-speed molding.
Under the circumstances, an object of the present invention is to provide a low-density polyethylene resin which can be molded by high speed molding or low temperature molding, in particular laminate molded at low temperatures and high speed without the above-described accompanying drawbacks upon extrusion laminate molding, and which has sufficient adhesion to a substrate, a composition thereof, and a laminate and production method for the laminate.
The present inventors have carried out intensive research on the above problems and as a result they have found that by use of a specified low-density polyethylene resin obtained by a high-pressure radical polymerization method or composition thereof, the conventional problems upon extrusion laminate molding, i.e., achieving a high speed of molding, low temperature molding and low temperature high speed molding, elimination of odor, and improved heat seal strength and adhesion strength, etc. can be solved, thus completing the present invention.
That is, the low-density polyethylene resin of the present invention is characterized by being obtained by high pressure radical polymerization method and has a density of 0.910 to 0.935 g/cm3, a melt flow rate of 0.1 to 300 g/10 min., and a terminal vinyl group number of 0.4 or more per 1,000 carbon atoms.
Also, the low-density polyethylene resin composition of the present invention comprises this low-density polyethylene resin to which less than 50 wt % of other polyethylene-based resins are blended.
The laminate of the present invention is characterized in that a layer of the low-density polyethylene resin or of the low-density polyethylene resin composition is provided on at least one surface of a substrate.
The method for producing a laminate according to the present invention is an extrusion laminate method which comprises a substrate supplying step for supplying a substrate, a resin surface treating step for practicing ozone treatment on a molten film consisting of a low-density polyethylene resin or low-density polyethylene resin composition of the present invention, a laminating step for pressing the ozone-treated surface of the molten film onto at least one surface of the substrate, and a winding step for winding the resultant laminate.
In the present invention, when low temperature molding where the resin temperature upon extrusion lamination is 200 to 310xc2x0 C., high adhesion strength can be obtained and as a result, the occurrence of smoking and odor can be inhibited.
Further, when high speed molding where the molding speed upon extrusion lamination is 200 m/min. or higher is adopted, high adhesive strength can be obtained so that the method of the invention is excellent in productivity and economy. Since reducing the thickness of the laminate is easy, it is excellent in productivity and economy.
When an ordinary molding temperature is used, the molding speed can be double the conventional speed, so that the productivity can be increased markedly.
In addition, the low-density polyethylene resin or composition thereof of the present invention is inexpensive and the method for producing the laminate is simple and easy.